1. Field of the Invention
This invention relates to ocean thermal energy conversion (OTEC). More particularly, it relates to the Cold Water Pipe that is used to bring colder water from depth to floating offshore plantships equipped with OTEC facilities.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Oceans cover somewhat more than 70 percent of the Earth's surface. This makes them the world's largest solar energy collector and energy storage system. On an average day, 60 million square kilometers (23 million square miles) of tropical seas absorb an amount of solar radiation equal in heat content to about 250 billion barrels of oil. If less than one-tenth of one percent of this stored solar energy could be converted into electric power, it would supply more than 20 times the total amount of electricity consumed in the United States on any given day.
Ocean thermal energy conversion (OTEC) is an energy technology that may be used to convert solar radiation to electric power. OTEC systems use the ocean's natural thermal gradient—the fact that the ocean's various water layers have different temperatures—to drive a power-producing cycle. If the temperature between the warm surface water and the cold deep water differs by at least about 20° C. (36° F.), an OTEC system has the potential to produce a significant amount of power. The oceans are thus a vast renewable resource, with the potential to produce billions of watts of electric power. The cold, deep seawater used in the OTEC process is also rich in nutrients, and it may be used to culture both marine organisms and plant life near the shore or on land.
Ocean thermal energy conversion (OTEC) processes utilize the difference between cooler, deep water and warmer water near the surface of the ocean to power a heat engine and produce useful work, usually in the form of electricity generation.
A heat engine achieves greater efficiency and power when run with a large temperature difference. In the oceans, the temperature difference between surface water and deep water is greatest in the tropics (although still a modest 20° C. to 25° C.). It is therefore in the tropics that OTEC offers the greatest possibilities. OTEC has the potential to offer global amounts of energy that are 10 to 100 times greater than other ocean energy options such as wave or tidal power and OTEC plants can operate continuously thereby providing a base load supply for an electrical power generation system.
The main technical challenge of OTEC is generating significant amounts of power efficiently from small temperature differences. It is therefore necessary to bring cold water up from depth with the minimum amount of power expended in order to achieve acceptable over-all efficiency in an OTEC plant.
The most commonly used heat cycle for OTEC is the Rankine cycle using a low-pressure turbine. Systems may be either closed-cycle or open-cycle. Closed-cycle engines use working fluids that are typically used as refrigerants such as ammonia and R-134a. Open-cycle engines may use vapor from the seawater itself as the working fluid.
An OTEC facility may also supply large quantities of cold water as a byproduct. This may be used for air conditioning and/or refrigeration and fertile deep ocean water may feed biological technologies. Another potential byproduct of an OTEC plant is fresh water distilled from seawater.
U.S. Pat. No. 4,231,312 describes an ocean thermal energy conversion facility having a cold water riser pipe that is releasably supported at its upper end by the hull of the floating facility. The pipe may be substantially vertical and has its lower end far below the hull above the ocean floor. The pipe is defined essentially entirely of a material which has a modulus of elasticity substantially less than that of steel, e.g., high density polyethylene, so that the pipe is flexible and compliant to rather than resistant to applied bending moments. The position of the lower end of the pipe relative to the hull may be stabilized by a weight suspended below the lower end of the pipe on a flexible line. The pipe, apart from the weight, may be positively buoyant. If support of the upper end of the pipe is released, the pipe sinks to the ocean floor, but is not damaged as the length of the line between the pipe and the weight is sufficient to allow the buoyant pipe to come to a stop within the line length after the weight contacts the ocean floor, and thereafter to float submerged above the ocean floor while moored to the ocean floor by the weight. The upper end of the pipe, while supported by the hull, communicates to a sump in the hull in which the water level is maintained below the ambient water level. The sump volume is sufficient to keep the pipe full during heaving of the hull, thereby preventing collapse of the pipe.
U.S. Patent Application Pub. No. 2009/0309271 describes a process and apparatus for multi-shot, liquid-resin-molding of continuous-fiber, composite articles. The process involves the step-wise fabrication of an article wherein continuity of the fibers is maintained between the multiple workpieces of the finished composite article. The system may be used for manufacturing and installing a single large diameter CWP that is made from continuous-fiber composite. This method includes the manufacture of the entire the CWP onboard the OTEC vessel by molding and lowering lengths of continuous-fiber composite during a 2- to 3-month period. Supply vessels transport resin to the vessel which initially acts as an offshore manufacturing plant and is subsequently transformed into an OTEC plant. The advantage of this method is that it may scale to the very largest diameters envisioned for OTEC plants and may produce a robust CWP with good thermal properties.
However, this system has several important drawbacks. In general, this is a radical departure from proven offshore riser technology with consequent uncertainties in cost, schedule, and feasibility.
The manufacturing phase requires low vessel motions. Low vessel motions suggest a semisubmersible vessel. In order to equip a semisubmersible vessel for OTEC service it may be necessary to introduce large supplementary buoyancy and stability modules. This supplementary buoyancy causes the semisubmersible to become less transparent to metocean conditions which increases its motions and may lead to fundamental structural engineering problems—e.g., the preservation of the vessel during tropical storms and ensuring that the many routine operation and maintenance tasks necessitating removal/re-attachment of the supplemental buoyancy do not lead to marine collision events.
During the two- to three-month period of time that the vessel is manufacturing the riser in the exposed location, it is necessary to maintain low motions on the manufacturing plant in order to practice high-quality molding operations and to ensure that the partially-formed, large-diameter, thin-wall CWP does not slip in its special gripping devices.